APPARENT DIGESTIBILITY OF DIETARY FIBER AND ITS COMPONENTS IN HUMAN SUBJECTS

The main objective of this research was to examine the fate of dietary fiber in the human gut. Apparent digestibility of fiber and its components was measured during a 2 month metabolic study. Seven healthy women consumed a low fiber diet for about 1 month and then the same diet for an additional month except that 16 g/day refined cellulose (Solka Floc) was added to the diet. The low fiber basal diet contained 3 servings of fruits, 4 of vegetables, and 1 each of rice and potatoes. Total water-insoluble fiber was measured by the neutral detergent fiber (NDF) method; cellulose was determined by the Crampton and Maynard procedure; and hemicellulose was estimated as the difference between NDF and cellulose.; During the low fiber diet, the apparent digestibility of all 3 fiber fractions was about 70%. When Solka Floc was added to the low fiber diet, digestibility of NDF, cellulose, and hemicellulose decreased significantly to 23%, 16%, and 51%, respectively.; The consequences of Solka Floc ingestion on energy metabolism also were determined. Solka Floc had no effect on fecal excretion of fat or nitrogen, but did increase energy in the feces. The increased fecal energy, though, was due to undigested refined cellulose. The potential energy available from fiber digestion during both diets was calculated and found to be less than 2% of the gross energy of the total daily intake.; Because the carbohydrate composition of a fiber may determine its physiological action, methods to hydrolyze fiber fractions and measure constituent monosaccharides were studied and optimized. The abilities of two high performance liquid chromatography (HPLC) columns, a normal phase column and a cation exchange column, to separate the monosaccharides in dietary fiber were evaluated. The heavy metal cation exchange column was selected as the better column for separation of the sugars in fiber hydrolysates, namely glucose, galactose, mannose, rhamnose, xylose and arabinose.; Saeman hydrolysis with a 3 hour primary hydrolysis and 2 hour secondary hydrolysis gave the best recovery of monosaccharides from NDF. Following hydrolysis, samples were neutralized, separated by HPLC, and detected by differential refractometry. Food NDF from both diets was greater than 80% glucose, while fecal NDF was 72% glucose during the low fiber diet and 87% glucose during the high fiber diet. Xylose was the next most common monosaccharide in all NDF residues, while all other monosaccharides were minor components of NDF.; The monosaccharides in 3 Crampton and Maynard residues from food and feces were analyzed by the same procedures used for the NDF residues. The cellulose residues were 85-87% glucose and 5% hemicellulosic sugars, xylose and arabinose. Because of the hemicellulosic contamination of the Crampton and Maynard residue, the difference between NDF and cellulose underestimated water-insoluble hemicellulose by between 6 and 17%.; The apparent digestibility of all the monosaccharides in NDF was significantly decreased when the Solka Floc was ingested: glucose from 83 to 40%, xylose 52 to 10%, galactose/rhamnose (which co-eluted) 73 to 38%, arabinose 86 to 72%, and mannose 86 to 50%. Xylose was significantly less digestible than all other monosaccharides during both diets.; In conclusion, neutral detergent fiber and Crampton and Maynard cellulose residues were both mainly glucose during the low and high fiber diets. Food-derived fiber, whether measured as NDF or the component monosaccharides in NDF, was extensively degraded during gut transit, while Solka Floc was a poor substrate for the gut microbes. Solka Floc caused an increase in fecal energy during the high fiber diet. But, the extra fecal energy was due to undigested refined cellulose and thus was of little physiological significance.